The invention relates to a method for registration correction for a multicolor rotary printing press in an acceleration phase, having at least one registration correction device with correction parameters for operation of constant speed, and having a motion controller for controlling the print roller speed.
In multicolor printing in rotary printing presses, the application of the individual color separations, especially for cyan, magenta, yellow, and black, is done in successive printing groups. The material to be printed is furnished in rolls and is guided in endless fashion through the printing unit. A criterion for the print quality achieved is that the printed images of the various colors be located exactly one above the other. This location of the printed images above one another is called registration. For mutual alignment with the various printing groups, in addition to the actual printed image, each printing group prints registration measurement marks, for example in the form of registration crosses. From these marks, an offset between the various printed images can be detected online by an optical measurement system. In rotary printing systems, this measurement system is generally a component of a correction system known as the registration correction system. The registration correction system intervenes in the printing process via suitable final control elements and compensates for the registration deviations detected by the optical measurement system. In particular, the web length of the material to be printed can be varied between successive printing groups by the final control elements in such a way that the printed images of successive printing units are located one above the other.
The cause of deviations among the various imprints are not only the relative position of the printing groups to one another but also changes in the geometry of the material being printed. These changes in geometry are caused for instance by the influence of moisture and by drying steps located between the printing groups.
During the printing process, the web tension is kept the same as much as possible, which makes good print quality without substantial corrections possible. The correction parameters of the registration correction system are adapted to this operating state. Conversely, in both positive and negative acceleration events, the web tension varies, with an adverse effect on the registration precision. This cannot be compensated for adequately by the correction system, which in this phase of operation leads according to rejection. Even after the acceleration phase, the correction system requires a certain time in operation in order to attain the appropriate controlling variables again. Once again, the result can be rejection.
In German Patent DE 40 37 728 C1, an apparatus for registration correction for multicolor rotary printing presses in the web/web method is described, with a corrector with a central unit for detecting all the correction parameters and the correction method and storing them in memory, with a scanner for detecting web registration marks, and with final control elements for correcting the lengthwise registration. The apparatus includes a monitoring unit, which has an acceleration detector that ascertains a change in speed of the printing press, deviating from a stable web speed, at the printing cylinders and which has a shutoff device that is in signal communication with the acceleration detector and which interrupts the correction activity upon the occurrence of a signal representing this change of speed, until a constant operating speed is again achieved.
A disadvantage of this apparatus is that to ascertain the acceleration, an acceleration detector, with at least one suitable speed indicator for detecting the web speed must be provided. Because of the additionally required components, this not only leads to increased system costs but also, because of the necessary cable connection of the speed indicator, to increased installation costs. The costs are increased still further because one such speed indicator must advantageously be provided at each printing group.
Another disadvantage is that often the available space inside the printing unit does not make it possible to mount the speed indicator in a suitable position.
Since during the change of speed the correction activity is interrupted, and the corrector output is fixed at the most recent value before the change of speed, no registration correction is performed during this phase. This leads to an increased registration deviation during the acceleration phase and thus to increased rejection.